Owing to the small structures (order of magnitude 1 .mu.m) currently required in semiconductor technology, photoresist structures are generated usually by high-resolution projection exposure systems. The positive resists which are used for their favorable characteristics are highly transparent to the radiation (.lambda.=435 nm) generally employed for projection irradiation, i.e., the photoresist in the irradiated regions is disintegrated relatively uniformly across its entire thickness. After the irradiated photoresist layers have been developed, which--to effectively control this step--is performed with diluted basic developer solutions, the holes generated in the photoresist have vertical walls. When the photo resist patterns thus produced are used as etch masks in a subsequent dry etch step, the shape of the holes in the photoresist pattern is accurately transferred to the etched material, i.e., the holes in the etched material have vertical walls, as well. When, in still further process steps, a layer is vapor-deposited on the etched structure, problems arise because the thickness of the deposited layer is non-uniform and too thin, particularly in the region of the hole walls and the hole edges, which leads to poor step coverage and tearing. A typical example of such phenomena are metal lines with weak spots along the edges of contact holes in isolating layers. The described defects are responsible for an increased failure rate of components containing such structures.
Measures are known that are taken to prevent holes in vertical walls in etched structures. It is known, for example, to use multi-step etching, wherein by stepwise expansion of the etch mask, steps are etched into the hole walls (contour etching). In practice, this leads to inclined hole walls which prevent the above-described difficulties during the deposition of a layer. However, this known process is time-consuming and complicated, and thus expensive and not readily reproducible.
In another known process, the walls of the holes in the photoresist are inclined by heating the photoresist structure after development, which softens the photoresist, causing it to flow. As a result, the photoresist edges are rounded and the hole walls are inclined, meaning that the dimensions of the hole at the bottom, where it opens towards the material to be etched, are smaller than at the top. However, this process has the disadvantge that the edge angles of the walls depend on the hole diameter and the packing density in the vicinity of the hole. The inclination of the hole walls in the photoresist pattern is transferred to the holes resulting in the etched material. Although much simpler than the previously described known process, the latter process is not sufficiently accurate for forming holes with very small dimensions that must be reproducible within narrow tolerances.
DE-OS 2 645 081 described a further process for producing photoresist patterns with inclined hole walls, in which
1. the UV-light used for irradiation defocuses or decollimates, and PA1 2. a small spacing has to be kept between the mask and the photoresist layer, and/or PA1 3. a thick (&gt;2 .mu.m) photoresist layer is used, and PA1 4. a disperse (non-collimated) light source is employed.
This process, however, has the disadvantage that the transferred pattern is poorly defined and not readily reproducible.
In EP application 0 227 851, another process for forming a photoresist pattern having holes with inclined walls is described, wherein regions of a layer of a positive photoresist are image-irradiated in a projection exposure system and, additionally, at least the photoresist regions adjoining the image-irradiated layer regions are irradiated, using a radiation at which the photoresist is highly transparent, and wherein the irradiated regions are developed by means of a basic developer. The additional irradiation may cover the entire photoresist layer or may be limited to the regions adjoining the image-irradiated regions. This process avoids the disadvantages described above, however, in its preferred embodiment the thickess of the photoresist layer is substantially reduced, which can only be avoided by using an additional mask which makes the process more complicated and more expensive.
A further method for influencing the wall profile of openings in photoresist layers is described in the article, "A Novel Method for Submicron Structurization Using Optical Projection Lithography," by K. Ismail, published in Microelectronic Engineering 1 (1983), page 295. In this method, called the "double exposure technique," the photoresist is exposed through a mask with the exposure time relatively short, such that the incident energy would not be high enough for developing the gate area. The mask is then shifted, for example, by using a micrometer screw, and exposed again for the same period where the shift is such that the two exposures overlap. The overlapping region is doubly exposed so that by developing the resist only this region is completely developed. The exposed opening has inclined walls. However, the angle range achievable of the wall inclination is restricted, especially when the ratio of the thickness of the resist layer to the dimension of the mask opening is large.
All the methods described above have in common that it is rather difficult, if not impossible, to shape the topography of the inclined walls in a predetermined manner. This option is very desirable. For example, if the photoresist pattern is used as a mask for doping a substrate by ion implantation, the doping profile in the substrate can be formed by shaping the wall profile in the photoresist pattern accordingly.
Sometimes it is of interest to tailor the openings in the photoresist layer with respect to their wall profile and/or with respect to their lateral shape, independently of the shape of the respective openings in the irradiation mask used. (In this context, lateral shape of an opening means the shape of the projection of the opening onto a surface aligned in parallel with the surface of the photoresist layer.) This applies especially to methods with which the reduction of the size of an opening in a mask can be achieved to a size below the resolution limit of the light used. The above cited article, published in Microelectronic Engineering, describes the formation of an opening in a photoresist layer which is narrower than the respective opening in the mask. However, the width of the opening is coupled to the inclination of its wall in the sense that with decreasing width, the inclination angle also decreases. Another method for achieving narrow openings in photoresist layers uses a bake cycle which causes the unexposed photoresist to flow into the developed opening whereby its size is reduced. However, this method is not sufficiently reproducible for application in VLSI technology.